A masked edit box is set to enter a date as "dd/mm/yyy"

mskBox1.mask = "##/##/####"

However, users allow to enter empty date or empty month like
"--/--/1990" or "--/09/1990", but not empty year. As mask is set as "##/##/####", only numbers are allowed.

How to reset mask in order to allow enter numbers and "--"?

Thank you.
vb999